The present invention relates to a method of manufacturing a heat exchanger having at least one longitudinally curved tube of flat cross-section which tube consists of two curved plates held spaced apart from each other by two curved edge strips all of which are held together by solder, the soldering together of the plates and strips being effected by a solder with which at least one of the mating surfaces between said plates and strips has been clad.
The object of the invention is to provide a method by which heat exchangers, having at least one curved tube of flat cross-section, can be manufactured in as simple convenient and economical manner as possible.
This object is achieved in accordance with the present invention by the use of a special solder having a predetermined range over which melting occurs. Accordingly, a straight tube of flat cross-section is first assembled from flat plates and flat edge strips by preliminary soldering together the component parts in a first soldering process using a first soldering temperature which lies in the lower part of the melting range of the special solder. The preliminary tube is then curved into the desired shape and is then permanently soldered together in a second or final soldering step using a soldering temperature which is within the melting range of the special solder but lies above the first soldering temperature. Preferably, a non-eutectic solder is used as the special solder in practicing the invention.
The term "non-eutectic solder" as used throughout this specification refers to solder which does not melt at a precisely defined melting point (melting temperature) but over a range which may extend over several degrees of temperature during which melting occurs. Within this melting range, the non-eutectic solder assumes different states of softness such that two bodies may be soldered together even at a temperature which lies in the lower portion of the melting range. The above described property of non-eutectic solder is due to the fact that when soldering at the lower limit of the melting-point range not all the solder is liquefied resulting in a solder joint having a reserve of unused solder. In accordance with the present invention, a second soldering process or step liquefies all the solder including the solder reserve, resulting in a stronger solder connection.
As a result of the two step soldering process of the present invention, straight plates and straight edge strips may be used for the first soldering step, thereby resulting in a surprisingly simple method of manufacture, since the subsequent curving of the flat cross-section tube produced in this manner can be carried out very simply by well known means. Using the present method, a longitudinally curved flat cross-section tube can be manufactured almost as easily as a straight flat cross-section tube consisting of flat plates and straight edge strips.
The invention therefore also makes it possible to conveniently manufacture heat exchangers having at least two curved flat cross-section tubes, as for instance, annular coolers or the like. This can be accomplished by assembling a package consisting of at least two flat cross-section tubes which have been soldered together in a first soldering step and then curved. Sheet metal fins are then arranged between the tubes which fins define straight flow paths perpendicular to the curved direction of flow through the flat cross-section tube. This package is then soldered together by the second soldering step.